1. Field of the Invention
The present invention generally relates to a converter switch circuit and coil configuration for a switched reluctance machine.
2. Description of Related Art
Many converter switch circuits have been designed for interfacing with switch reluctance machines. Switched reluctance machines (SRM) may require a large driving current based on the application and performance parameters of the SRM. If the SRM requires a high current draw, special high current electronic components must be used. Often, the high current components must be located on a separate board from low power electronics to minimize radio frequency interference and provide for proper heat dissipation.
One solution for providing higher current flow to switch reluctance machines while utilizing low power electronic components includes using several smaller discrete components in a parallel configuration to provide sufficient current flow to operate the switched reluctance machine. The parallel configuration allows the use of more commercially available components and reduces the overall cost of the electronics. In addition, the heat dissipation can be spread across multiple components allowing for a shared circuit board between the converter switch circuit and other low power electronics. Further, smaller parallel power switches provide better flexibility to integrate the motor and converter in one enclosure to provide improved space optimization.
However, one problem encountered with parallel power switches is that current sharing problems may arise. Even with matching the characteristics of the power switches, the power switches may not turn on or off at exactly the same time. The switching delay between the parallel power switches forces one of the power switches to carry much more than the maximum rated current during the delay time. The current through the switch that turns on earlier will be at least twice the normal current. This will cause more heat on the early power switch and will eventually damage the switch. The unbalanced sharing of current between parallel power switches, even for a short time, may cause power switch failures and ultimately destroy the converter itself. The damage of the first power switch overloads the next switch in parallel, and so on, creating a chain reaction until the whole converter is destroyed. Breakdown may be stopped, if the fault can be detected and the converter can be shut down very quickly. However, it is very difficult to detect the fault and shut off the converter in time. Another way to prevent the chain reaction breakdown of the power switches is to choose oversize components and heat sinks. However, using oversized components negatively affects cost and assembly complexity.
In view of the above, it is apparent that there exists a need for an improved converter switch circuit for a switched reluctance machine.